Yeah, as with most generations, the newer one just has more stuff to do in it. It has tougher Pokemon, more Pokemon, and a slightly different and better storyline IMO. I didn't find anything wrong with the storyline with diamond or pearl, but platinum is better. As mentioned before, I hate most of the new Pokemon based on purely liking them. A lot of them are very useful, but I don't like what they look like. My example is Bronzong. If you take away the two side parts, he looks like a freaking toilet. Here's my example.
Does this not look like a weird totem pole toilet to you?
I do really like Staraptor, Torterra, Bastiodon, Vespiquen, Gastrodon, Spiritomb, Garchomp, Lucario, Drapion, Hippowdon, Rhyperior, and Gallade. That is quite a bit. And I guess that i think all the starters are all right, but as you can see, I don't really have much for Grass other than Torterra. Final moveset is usually like Leaf Storm, Earthquake, Crunch, and Stone Edge. Empoleon usually has like Surf, Steel Wing, Drill Peck, and Earthquake, and Infernape has Close Combat, Mach Punch, Blast Burn, and Flame Wheel.